Cathode ray tube having metallic layer of non-uniform thickness



, Sept? 1967 YAsLmansu WATANABE ETAL 3,340,421

CATHODI RAY TUBE HAVING METALLI-C LAYER OF' NON-UNIFORM THICKNESS FiledJuly 30, 1964 elecran en? r-gy fnzzsnT'mrfs. )zsu ml''SLL Wafalabe Taka.'L'

/noue United States Patent O 3,340,421 CATHODE RAY TUBE HAVING METALLICLAYER OF NON-UNIFORM THICKNESS Yasumitsu Watanabe and Takuji Inoue,Tokyo, Japan, assignors to Sony Corporation, Tokyo, Japan, a corporationof Japan Filed July 30, 1964, Ser. No. 386,349 Claims priority,application Japan, July 31, 1963, 38/41,203 Claims. (Cl. 315-14)ABSTRACT OF THE DISCLOSURE A cathode ray tube of the post deflecting andfocus- This invention relates and more particularly to a post deflectingand focusing (PDF) type cathode ray tube. l

In ordinary cathode ray tubes a metallic layer which is over-laid -on afluorescent screen is provided with a uniformity in its thickness. As aresult of the thickness uniformity, brightness of the fluorescent screenis subtantially constant over the entire area thereof.

However, in .cathode ray tubes, such as a chromatrontype cathode raytube having a grid for post deecting and focusing action disposed inspaced relation to the screen, some emitted electrons from the electrongun will impinge upon the grid structure causing emission of secondaryelectrons therefrom. The probability that the emitted electrons willimpinge upon the grid increases at .the peripheral portions Whereincidence angles of electron beams are large with respect to the gridstructures, and hence, secondary electrons from the grid inevitablyincrease greatly. Accordingly, when the thickness of the metal backlayer on the uorescent screen has been made uniform, a so-called colorrunning occurs at the peripheral portions of the screen. This effect isparticularly noticeable at the two sides of a cathode ray tube formingvertical areas -of color running. This means that the contrast ratio isreduced which depends upon the primary electron, namely the scanningelectron beam and the secondary electron.

A principal object of this invention is to provide a PDP-type cathoderay tube.

Another object of this invention is to provide a PDF- type cathode raytube in which the aforementioned color-running or fog at the peripheralportions of the screen may effectively be avoided.

A further object of this invention is to provide an improved colorcathode ray tube.

Other objects, features and advantages of this invention will beapparent from the following detailed description when taken inconjunction with the accompanying drawings, in which:

FIGURE 1 is a graph, illustrating the transmission factor through ametallic layer in relation to the electron energy and layer thickness;

FIGURE 2 is a sectional side elevational view schematically illustratinga preferred embodiment of a PDF- type cathode ray tube according to thisinvention; and

FIGURE 3 is a partial sectional view of one corner portion of thecathode ray tube illustrated in FIGURE 1.

Like reference numerals throughout the various views of the drawings areintended to designate the same or similar structures.

generally to a cathode ray tube l FIGURE 1 illustrates va family ofcurves of electron energy transmission factors, with a thickness d of ametal back layer, for example, an aluminum coating layer, as aparameter. As is readily apparent from FIGURE 1, when the thickness d ofthe metallic layer is relatively large, the transmission factor is smalland a small energy electron beam cannot be transmitted. The presentinvention is intended to utilize this influence to reduce the effects-due tosecondary electron emission at the peripheral portions of ascreen.

With reference to FIGURES 2 and 3, there is shown one preferredembodiment of the present invention. A PDF-type cathode ray tube, forexample, a chromatrontype tube, generally designated with the referencenumeral 1 includes a line grid 2 to which switching signals for colorselecting are supplied as is well known in the art. A phosphor screen 3of the cathode ray tube 1 includes a repeating array of red, blue andgreen vertical stripes 4. A metallic layer 5 such as aluminum or thelike is coated on the inside surface of the screen. A11 electron beamsource such as for example an electron gun device 6 is disposed foremitting electrons therefrom. The emitted electrons are directed towardthe line grid 2 as illustrated by the electron path generally designatedWith the reference numeral 7.

It is to be understood that in FIGURES 2 and 3, parts which have nodirect relationship with respect to the explanation of the presentinvention, such as for example, an electron beam delecting device andthe like, are omited for the sake of simplicity.

In the present invention, the thickness of the metallic layer 5 isdifferent at a peripheral portion 5a than that at a central portion 5bthereof. That is, a thickness d1 of the peripheral portion 5a is madegreater than a thickness d2 of the central portion 5b. The thickness ofthe metal back layer 5 increases gradually o-r with offset toward theperipheral portion thereof from the center axis of the electron beamsource l6. That is, the thickness increases directly with the radialdistance from the center of the tube to the periphery of the tube.

As a result of this change in thickness dimension, the transmissionfactor at the peripheral portion 5a of the screen 3 is reduced toprevent the secondary electrons from passing through the metal backlayer 5. It must be noted that the energy of the secondary electron isgenerally smaller than that of the primary electron.

According to the present invention, the 'transmission factor of themetallic layer S corresponding to the peripheral portion of the grid 2where the secondary electrons are produced at a high rate, can becontrolled by varying the thickness of the layer 5. In this case,luminescence due to the secondary electron at the peripheral portion ofthe screen 3 is reduced, so that the contrast ratio of the primaryelectron, namely the scanning electron beam vs. the secondary electronat the peripheral portion may be made substantially equal to that at thecentral portion. Therefore, the color running, fog and the like at theperipheral portion of the screen can be avoided or greatly decreased.Thus, the present invention has great advantages in providing higherquality picture representation.

One example -of the present invention includes operating the cathode raytube with a voltage of approximately 25 kilovolts on the metallic layerand an operating point of the grid at 6 kilovolts. A thickness ofapproximately 5,000 A. was employed for the dimension d2 and aIthickness `of approximately 10,000 to 15,000 A. was employed for thedimension d1.

It will be apparent that many modifications and variations may beeffected without departing from the scope of the novel concept of thisinvention.

What is claimed is:

1. A cathode ray tube comprising an electron gun,

a screen portion,

a lluorescent material on said screen portion,

a grid located adjacent to but spaced from said screen,

and

a metallic layer overlying said fluorescent material,

the thickness of the metallic layer at a peripheral portion thereofbeing larger with respect to the thickness at a central portion thereof.

2. The cathode ray tube of claim 1 including an electron energy sourceof approximately 25 kilovolts connected to said metallic layer, and thethickness of the peripheral portion and the central portion of themetallic layer being in the range of approximately 10,000 A. to 15,000A. and approximately 5,000' A. respectively.

3. A cathode ray tube comprising an electron gun,

a screen portion,

a fluorescent material on said screen portion,

a grid located adjacent to but spaced from said screen,

and

a metallic layer overlying said uorescent material,

thethickness of the Imetallic layer at two edges 25 portion thereofbeing larger with respect to the thickness at a -central portionthereof.

4. A cathode ray tube comprising an electron gun,

a screen portion,

a fluorescent material on said screen portion,

a grid located adjacent to but spaced from said screen,

and

a metallic layer overlying said uorescent material,

and having a thickness increasing from a central portion to a peripheralportion thereof.

having a thickness increasing directly with the radial distance from thecenter thereof.

References Cited UNITED STATES PATENTS 11/1959 Evans 313-78 AssistantExaminers.

1. A CATHODE RAY TUBE COMPRISING AN ELECTRON GUN, A SCREEN PORTION, AFLUORESCENT MATERIAL ON SAID SCREEN PORTION, A GRID LOCATED ADJACENT TOBUT SPACED FROM SAID SCREEN AND A METALLIC LAYER OVERLYING SAIDFLUORESCENT MATERIAL, THE THICKNESS OF THE METALLIC LAYER AT APERIPHERAL PORTION THEREOF BEING LARGER WITH RESPECT TO THE THICKNESS ATA CENTRAL PORTION THEREOF.